Trucks marked as being from a bitterly disputed Russian aid convoy to Ukraine stand in line as they return to Russia on Aug. 23, 2014. / Alexander Roslyakov, AP

by Doug Stanglin and Michael Winter, USATODAY

by Doug Stanglin and Michael Winter, USATODAY

Hundreds of trucks in a disputed Russian aid convoy to Ukraine crossed back into Russia on Saturday, helping to ease tensions in the region.

Paul Picard, head of the border observation mission for the Organization for Security and Cooperation in Europe (OSCE), said all vehicles that had crossed into Ukraine had returned to Russia by mid-afternoon. A Russian emergency official said 227 vehicles had taken part.

An Associated Press reporter on the Ukrainian side of the border was able to look inside about 40 of the white-tarpaulined tractor-trailers and confirmed they were empty. However, a spokesman for Ukraine's military, Col. Andriy Lysenko, said the trucks were loaded with military equipment before heading back to Russia, according to a report by The Washington Post.

German Chancellor Angela Merkel held talks with Ukrainian President Petro Poroshenko in Kiev on Saturday and promised 500 million euros ($660 million) in loan guarantees to support private investment in infrastructure and schools in war-struck areas.

Merkel urged a political solution to the crisis three days before Poroshenko will be meeting Russian President Vladimir Putin in Minsk in their first encounter since June. Merkel said she was looking forward to the outcome of those talks and expressed "hope that at least a step forward will be reached there."

"There must be two sides to be successful. You cannot achieve peace on your own. I hope the talks with Russia will lead to success," Merkel said at a Saturday news conference after meeting with Poroshenko. The German chancellor called for an agreement that would allow the border to be monitored by the OSCE, Reuters reported.

Poroshenko said that while Ukraine wants peace, it cannot be "at the price of sovereignty, territorial integrity and the independence of Ukraine."

The developments come after a tense day Friday, when the aid convoy carrying "humanitarian cargoes" defied the Ukrainian government and International Committee of the Red Cross by crossing the border and arriving in Luhansk, the separatists' stronghold in eastern Ukraine.

NATO Secretary General Anders Fogh Rasmussen condemned Russia for sending the unauthorized humanitarian aid convoy into Ukraine on Friday without the involvement of the International Committee of the Red Cross. On Saturday, the Russian foreign ministry called Rasmussen's statement "another lie," Russian news agency RIA Novosti reported.

The Kiev government had called the advance of the Russian trucks into Ukrainian territory on Friday a "direct invasion."

U.S. Deputy National Security Adviser Ben Rhodes warned Russia to remove the convoy from Ukraine or "face additional costs and consequences" from the United States and its allies.

The controversial Russian aid convoy left Moscow on Aug. 12 with 2,000 tons of humanitarian aid, including baby food, medicine, grain, sugar, sleeping bags and other items, Russia's government-run RIA Novosti reports. The vehicles, covered with white tarps, had been stuck at a border crossing in a dispute between Russia and Ukraine over inspection of the vehicles.

The Russian foreign ministry, in a statement on its website, said Russia took unilateral action to get the convoy moving and accused Ukraine of deliberately delaying the aid. "The Russian side has decided to act," the foreign ministry said.

Meanwhile, NATO officials said Friday that Moscow has sent Russian-manned artillery units into Ukraine in recent days and was using them to shell Ukrainian forces as part of a "major escalation" of Russian involvement in the disputed region.

"Russian artillery support - both cross-border and from within Ukraine - is being employed against the Ukrainian armed forces," Rasmussen said, adding large quantities of advanced weapons, including tanks, armored personnel carriers and artillery had been transferred recently to separatist groups in eastern Ukraine.

"Moreover, NATO is observing an alarming build-up of Russian ground and air forces in the vicinity of Ukraine," Rasmussen said.

Rhodes also condemned the firing of artillery from Russia into Ukraine and a "disturbing movement of Russian artillery and military equipment" into the country.

The warnings come as Ukrainian troops have begun to gain ground against Russian-backed separatists in their battles over the disputed area since April.